The invention relates to a synchronization control method for a data transmission system utilizing an orthogonal frequency division multiplex and its transmission system.
In recent years, as a signal multiplex system for a digital radio communications for mobile terminals or terrestrial systems, attention has been paid to an orthogonal frequency division multiplex (hereinafter, referred to as an OFDM system) having a feature such that it is strong against a multipath fading or ghost.
According to such a system, an information code is transmitted by using a signal obtained by digitally modulating tens to hundreds of kinds of a number of carrier waves arranged with a same frequency interval fs by a symbol frequency fsy (=1/Tsy), namely, by using an OFDM signal (orthogonal frequency division multiplexed signal).
In the case where a transmission signal which was modulated by such a system and transmitted is received and demodulated on a reception side, first, it is necessary to obtain synchronization from the received OFDM signal.
In this case, the operation to obtain synchronization of the OFDM signal denotes that a head position of a data symbol is detected from the OFDM reception signal by a receiver apparatus and a demodulating process is started in accordance with a timing of the head position. To obtain the synchronization, therefore, it is necessary to obtain a reference signal showing the timing of the head position of the data symbol.
For this purpose, there has been proposed a method whereby, on a transmission side, a null interval serving as a non-signal period and a sync symbol group such as a sweep signal or the like having a signal component which changes from the highest frequency to the lowest frequency of a transmission band for a predetermined period are previously inserted to the head of a transmission frame serving as a unit of a data transmitting process and they are detected and synchronization is obtained on a reception side (xe2x80x9cA Study or Field Pickup Unit using OFDM Modulation Schemexe2x80x9d, Technical Report of The Institute of Television Engineers of Japan, Vol. 19, No. 18, issued on August, 1995). As an example of a specific method for the detection of the null interval and the clock synchronization using the sweep signal, there is the invention disclosed in U.S. patent application Ser. No. 09/203,564 filed on Dec. 2, 1998, according to the invention of the same applicant as that of the present invention.
The method of obtaining synchronization of the null interval and the OFDM signal having no sweep signal has been disclosed in U.S. Pat. No. 5,602,835 registered on Feb. 11, 1997. In this method, as mentioned later, a transmitted OFDM signal includes a time-axis base data signal obtained by OFDM-modulation of one symbol signal and a guard interval added to a head of the time-axis base data signal, wherein the guard interval is produced by copying a predetermined tail part of the time-axis base data signal. According to the method, a calculation for obtaining a mutual correlation value, that is a degree (intensity) of correlation between two signals, of the OFDM reception signal and the signal obtained by delaying the reception signal by one effective symbol is implemented. The effective symbol means a single symbol without the guard interval. According to the method, since there is a delay of one effective symbol period between the received signal and the delayed signal, a point where a data signal for a predetermined period at the end of a data symbol of the reception signal and a guard interval added at the head of the data symbol of the delay signal coincide on the time axis, and a correlation value becomes the maximum value is obtained. The demodulating operation of the reception signal is performed by setting a position on a time base at the time of obtaining the maximum value to a reference.
A method of obtaining synchronization of an OFDM signal by using a sync symbol group will be briefly explained hereinbelow with reference to FIG. 3.
FIG. 3 shows a synchronization detecting unit of a demodulating unit on a receiving unit side of a digital data transmission system in which a transmission signal in which null intervals have been inserted at regular periods is received, an electric power value of the reception signal is obtained, a magnitude of the obtained power value is discriminated by a comparator, the null interval is detected, and synchronization with the reception signal is obtained.
An RF transmission signal of the OFDM system which has been transmitted from a transmitter Tx and in which the null intervals have been inserted at regular periods is received by a receiver Rx, the RF signal is converted into a baseband signal S21 by a down-converter 21 of the receiver Rx, and a digital reception signal obtained by digitally converting the signal S21 by an A/D converter 22 is supplied to a terminal 1. An instantaneous power value of the digital reception signal supplied to the terminal 1 is obtained by a power calculator 15.
An average power of a power value S11 outputted from the power calculator 15 is obtained by an average power calculator 6. The average power is delayed by time corresponding to one symbol or more by a delay 7. A multiplier 9 increases an output (average power) of the delay 7 by 1/N time (N is a positive real number), thereby obtaining a threshold value S13 for comparing with the power value S11.
The level of the power value is discriminated by a comparator 12 of an adaptive type reception level discriminator 14. When the power value S11 is larger than the threshold value S13, an output S12 of the level discriminator 14 is set to the xe2x80x9cHigh (H)xe2x80x9d level. If S11 is smaller than the threshold value, the output S12 is set to the xe2x80x9cLow (L)xe2x80x9d level. Since the output itself of the adaptive type reception level discriminator 14 is used merely for discriminating the magnitude of the reception signal as mentioned above, whether the xe2x80x9cLxe2x80x9d level continues for a predetermined length (time) or not is not discriminated.
In a null interval discriminator 19, therefore, in the case where a state in which the output of the reception level discriminator 14 is at the xe2x80x9cHxe2x80x9d level continues for the predetermined length (time), it is determined that a null interval exists, and a null interval detection pulse S19 is generated.
By the construction as mentioned above, the null interval during which the xe2x80x9cHxe2x80x9d level or the xe2x80x9cLxe2x80x9d level continues for the predetermined length (time) is detected from the reception signal, thereby enabling a synchronizing position of a frame start point to be coarsely matched.
However, in order to more correctly demodulate the reception signal by the receiver Rx, in the receiver Rx, a count start point (start point of sampling a data symbol in a demodulator 40) of a frame counter 24 of the receiver Rx needs to be matched up to a precision of a 1-clock period based on the received reception signal.
As a method of accomplishing the above object, in the Transmitter Tx, besides a null symbol, a sync symbol for showing a specific time point on a time base is inserted into a transmission signal to be transmitted.
As a sync symbol to be inserted, there is a sweep signal, a PN code, or the like which changes from the predetermined highest frequency to the lowest frequency.
A case of using the baseband signal S21 in which a sweep symbol (sync symbol) has been inserted subsequently to the null symbol will be described hereinbelow with reference to a signal diagram of FIG. 4. A frequency component included in the sweep symbol of the baseband signal S21 is shown at (q) in FIG. 4.
A correlation between a reference signal (the same sweep signal as that of (q) in FIG. 4) which is equivalent to a frequency pattern of a sweep signal set in the receiver Rx in a sweep correlation arithmetic operating device 2 in FIG. 3 and the received baseband signal S21 shown (p) in FIG. 4 is arithmetically operated. The sweep correlation arithmetic operation ranges at k=0 and k=14 are shown in FIG. 4 as arithmetic calculation windows for correlation.
As shown in FIG. 4, according to such a correlation arithmetic operation, a peak of a correlation value in the 1-symbol period is detected while a sampling point to start the correlation arithmetic operation is sequentially shifted by a 1-clock period at a time.
For example, assuming that the number (k) of times of the correlation arithmetic operation is set to 15 and, each time the start point of the correlation arithmetic operation is sequentially shifted one by one (k=1 at a time) from k=0 to k=14, the result of the correlation arithmetic operation is plotted, the result is as shown at (r) in FIG. 4. An axis of abscissa denotes a sampling point and an axis of ordinate indicates a correlation value. FIG. 5 is an enlarged diagram of (r) in FIG. 4.
In the example, it is shown that the maximum correlation exists at the seventh sample (k=7) from the correlation arithmetic operation start point.
The null interval detection signal S19 which is generated when the null interval is detected by the null interval discriminator 19 in FIG. 3 is inputted to a counter 27 for adjusting the timing for starting the sweep correlation arithmetic operation, and a counter value is cleared.
In a comparator 26, when a count output S27 of the counter 27 reaches a value set by a constant register 28, a correlation arithmetic operation start signal S26 is generated. The signal S26 is used as a correlation arithmetic operation start timing of the sweep correlation arithmetic operating device 2.
Whether the value of the peak of the correlation arithmetic operation calculated by the correlation arithmetic operating device 2 has significance or not is subsequently discriminated. A method of discriminating the significance will be described with reference to FIGS. 3 and 5. The value of the correlation arithmetic operation is proportional to the level of the reception signal.
In FIG. 3, the baseband reception signal is subjected to the foregoing sweep correlation arithmetic operation by the sweep correlation arithmetic operating device 2, so that a sweep correlation value 124 is obtained. The sweep correlation value 124 is shown at C1 in FIG. 5.
Since the sweep correlation arithmetic operation is performed while the sampling point is shifted one by one from the correlation arithmetic operation start point, both the sweep correlation value 124 and the number of arithmetic operating times 125 showing how many times the correlation arithmetic operation has been performed at the time of the value 124 (the number of times is shown by k) are outputted from the sweep correlation arithmetic operating device 2.
In a peak discriminator 17 of the sweep correlation, a magnitude of the maximum value of the sweep correlation value 124 is discriminated, thereby discriminating whether the sweep correlation value has significance or not.
A threshold value which is used for the significance discrimination about the magnitude of the maximum value of the sweep correlation value is obtained by using a delay reception signal power value S7 which is obtained by delaying the output of average power calculator 6 by the delay 7 and level converting it by a multiplier 8. That is, this threshold value is determined on the basis of the average power of the reception signal.
The reason why the threshold value is varied is because the result of the sweep correlation arithmetic operation changes in proportion to the level of the reception signal.
That is, this is because when the reception signal is at the standard level, even if C4 in FIG. 5 is suitable as a threshold value, if the level of the reception signal fluctuates and decreases, C5 in FIG. 5 is more suitable than C4.
An output S17 of the peak discriminator 17 shows a value at a sampling point k at which the correlation peak determined to have the significance was obtained. The signal S17 indicative of the position of the correlation peak is inputted to an adder 29. A value corresponding to a half of total number of correlation arithmetic operation has been preset in a constant register 30. In case of the embodiment, the value in the constant register 30 is equal to xe2x80x9c7xe2x80x9d which is a half of 15 that is the total number of correlation arithmetic operation. In the adder 29, the value of the constant register 30 is compared with the signal S17 indicative of the position of the actual correlation peak and a timing correction signal S29 according to a difference between them is generated. The correction signal S29 shows by which amount the actual correlation peak position in the reception signal is deviated from the specified value (value of the constant register 30).
A counter 23 is a counter for correcting a reset timing of the frame counter 24. The counter 23 is cleared by the null interval detection signal S19 and starts the up-counting operation.
An output of the counter 23 is compared by an comparator 25 with the frame counter reset timing correction value S29 obtained by adding the correlation peak position signal S17 and the value of the constant register 30 by the adder 29. When they coincide, the counter 23 generates a frame counter reset signal 4.
The frame counter 24 is cleared by the frame counter reset signal 4 and generates a control signal S24 for the receiver Rx. The frame counter reset signal 4 presents a demodulation start point of the demodulator 40. That is, it is a reference signal for deciding the sampling start point of the data symbol in the received signal.
In case of transmitting data by using a radio transmission path such as space or the like, the receiver Rx receives a transmission signal which has a multipath fading and in which besides the transmission signal itself (hereinafter, referred to as a principal wave) which directly arrives from the transmitter Tx, a delayed transmission signal (hereinafter, referred to as a reflection wave) generated as a result of that the transmission signal is reflected by a mountain, a building, or the like has been synthesized.
As for the transmission signal having the multipath fading, since the principal wave and the reflection wave are synthesized on the transmission path, if the delay wave (reflection wave) of the principal wave is added to the transmission signal (principal wave) in which the sweep symbol has been inserted, as shown in FIG. 6A, in addition to a peak C1 of the principal wave, a peak C6 due to the reflection wave is caused as a result of the correlation arithmetic operation.
Since the multipath fading changes with the elapse of time, as a result of the correlation arithmetic operation, each peak successively changes as shown in FIGS. 6B and 6C.
In FIGS. 6A, 6B, and 6C, C4 denotes the threshold value for discrimination about the significance of the correlation arithmetic operation result.
In such a situation, in the case where the sync detection is performed from the reception signal by the receiver Rx shown in FIG. 3, the frame timing as an output of the counter 24 of the receiver is adjusted so that the maximum value (value exceeding the threshold value for discrimination about the significance) of the correlation arithmetic operation result is matched with the position of the reference timing (k=7 which is the intermediate value of the total number of correlation arithmetic operations) of the receiver.
As shown in FIGS. 6A and 6C, when the signal having the multipath fading as mentioned above is first received by the receiver as a principal wave peak C1 of the value exceeding the threshold value C4, the principal wave can be matched with the reference timing k=7 of the receiver.
However, as shown in FIG. 6B, in the case where the principal wave peak C1 does not exceed the threshold value C4 but the peak C6 of the reflection wave has a value exceeding the threshold value C4, a problem such that it is erroneously regarded that the reflection wave is a signal serving as a reference for synchronization, the reflection wave is matched with the reference timing k=7 of the receiver as shown in FIG. 7A, and the receiver is synchronized with the reflection wave.
It is an object of the invention to solve the above problems and to provide a data transmission system of the OFDM system which can decrease probability of synchronization with the reflection wave via a multi-path and increase probability of synchronization with a principal wave and perform a stable synchronization detection even in a situation where a multipath fading exists and to provide a method for such a system.
According to a data transmission system utilizing an orthogonal frequency division multiplex of the invention, a signal in which predetermined sync symbols have been inserted between data symbol groups at predetermined intervals is transmitted. According to a synchronization control method of the invention, the synchronization is obtained by the following steps in a receiver apparatus. First an arithmetic calculation is executed for obtaining a degree of mutual correlation of a reception signal which was orthogonal frequency division multiplexed and a predetermined sync symbol pattern a predetermined number of times in a predetermined arithmetic calculation window. The correlation values obtained after a predetermined number-th arithmetic calculation from the intermediate of the total number of arithmetic calculations are subjected to a predetermined reduction process. The maximum value is detected from a correlation value obtained in the predetermined former period and the adjusted correlation value in the predetermined latter period. A reference signal to demodulate the reception signal is formed on the basis of information of a time base position of a signal at the time when the maximum value is obtained.
Further, according to a data transmission system utilizing an orthogonal frequency division multiplex of another aspect of the invention, a signal in which data symbols to which guard intervals have been added are coupled is transmitted. According to a synchronization control method of the invention, synchronization is obtained in a receiver apparatus in the following steps. An arithmetic calculation is executed for obtaining a degree of mutual correlation of a reception signal which was orthogonal frequency division multiplexed and a signal obtained by delaying said reception signal by one effective symbol period a predetermined number of times in a predetermined arithmetic calculation window. The correlation values obtained after a predetermined number-th arithmetic calculation from the intermediate of the total number of arithmetic calculations are subjected to a predetermined reduction process. The maximum value is detected from a correlation value obtained in the predetermined former period and the adjusted correlation value in the latter period is detected. A reference signal to demodulate the reception signal is formed on the basis of information of a time base position of a signal at the time when the maximum value is obtained.
Thus, since a process for forcedly reducing a correlation arithmetic operation or calculation value of a reflection wave which is generated with a delay from a principal wave due to a multipath is performed, the correlation value of the reflection wave can be consequently set to be smaller than a threshold value for discriminating significance of a peak value of the correlation arithmetic operation value. Therefore, the problem such that the reference timing of the receiver is matched by the reflection wave and synchronized with the reflection wave does not occur. That is, even in a situation where the multipath fading exists, a confidence degree at which the receiver is synchronized with the principal wave can be improved. The confidence degree at which the receiver is synchronized with the principal wave is also improved even against the existence of the reflection wave having a long delay time without increasing the correlation arithmetic operation amount, and the synchronization can be stably and quickly obtained.